The invention relates to a method for making water-soluble nanocrystals, and, more particularly, to a method for making ordered nanocrystal arrays and subsequent nanocrystal mesophases through self-assembly of water-soluble nanocrystal-micelles.
Nanometer-sized crystallites of metals, semiconductors, and oxides exhibit optical, electronic and chemical properties often different from those of the corresponding isolated molecules or macroscopic solids. The ability to adjust properties through control of size, shape, composition, crystallinity, and structure has led to a wide range of potential applications for nanocrystals (NCs) in areas like optics, electronics, catalysis, magnetic storage, and biological labeling. Furthermore NC assembly into two- and three dimensional arrays is of interest for development of synthetic solids with collective optical and electronic properties that can be further tuned by the nanocrystal spacing and arrangement. For example, using uniformly-sized CdSe nanocrystals passivated with a close-packed monolayer of organic coordinating ligands (trioctylphosphine oxide or alkane-amines or mixtures thereof), monodisperse lyophobic colloids self-assembled to create periodic, three-dimensional quantum dot superlattices. In another instance, ordered CdSe quantum dot films were prepared, demonstrating optical gain and stimulated emission. In two-dimensional (2-D) quantum dot monolayers formed in a Langmuir trough, quantum mechanical tunneling was reported to occur between adjacent silver nanocrystals at an interparticle spacing below 1.2-nm and a reversible insulator-to-metal transition below 0.5-nm. Even richer transport and collective phenomena are expected for three-dimensional (3-D) NC arrays.
Despite recent advances in the synthesis and characterization of nanocrystals and nanocrystalline arrays, there remain numerous challenges that limit their practical utilization. For example, synthesis procedures generally used for metallic and semiconducting nanocrystals employ organic passivating ligands, making the nanocrystals water insoluble. This is problematic for biological imaging and for incorporation of nanocrystals in hydrophilic sol-gel matrices like silica or titania needed for the fabrication of robust, functional lasers. Furthermore, while steric stabilization of nanocrystals with organic passivating layers suppresses strong, attractive particle-particle interactions, thereby facilitating self-assembly of nanocrystal arrays, it necessarily causes the arrays to be mechanically weak and often thermally and chemically unstable. This ultimately limits routine integration of NCs into devices.
Three different approaches have been used to prepare water-soluble semiconductor NCs using currently-available hydrophobic NCs: 1) ligand-exchange, 2) encapsulation into a water-soluble shell (for example, silica or phospholipids) and 3) arrested precipitation in water. The ligand exchange method involves the use of carboxylic acid or amine terminated thiols (for example, amino-ethanethiol and mercaptopropionic acid) to replace trioctylphosphine or alkaneamine ligands. This often results in aggregation of resulting water-soluble NCs and a lower photoluminescence (PL) quantum efficiency (QE) (QE, 10-30%) than original NCs. In the case of CdSe NCs, the emission has been observed to be completely quenched after transfer into water, if no shell of a wider band gap material is used. The encapsulation of the NCs into a water-soluble shell typically yields PL QEs of 20-30%. The method of forming hydrophilic silica shell for transferring NCs into water involves several steps and thus has the additional disadvantage of being rather complicated and time-consuming. Arrested precipitation in water in the presence of stabilizers (for example, thiols) is a faster and simpler method to synthesize water-soluble quantum dots (QDs) and has been applied to several semiconductors potentially relevant to biolabeling (for example, CdS, CdSe, CdTe).
Useful would be a direct synthesis method of water-soluble nanocrystalline micelles (NC-micelles) utilizing hydrophobic compounds and their further formation into robust, ordered three-dimensional nanocrystalline mesophases in bulk or thin film forms.